The present invention relates to switching mode DC to DC converters and, more particularly, to DC to DC boost or buck converters utilizing synchronous rectification.
DC to DC converters are employed in virtually all electronic devices requiring a stable power supply potential. In general, a DC (Direct Current) potential is supplied to the electronic device from either a battery or an Alternating Current (AC) rectification circuit. The DC potential is generally operating at a level which is either lower than or higher than the voltage level required by the electronic device. Boost converters are employed in applications where the electronic devices require a higher operating voltage than is supplied by the battery or the AC rectification circuit. Conversely, buck converters are employed in applications where the electronic devices require a lower operating voltage than is supplied by the battery or the AC rectification circuit.
The rectification element is applied within the boost and buck converters to allow uni-directional energy flow from the battery or the AC rectification circuit to the electronic devices. Some implementations of DC to DC converters employ a Schottky diode as the rectification element and a power transistor is generally used for the main current switch. The Schottky diode rectifiers prevent any reverse current being conducted from the load into the converter, but generally have a high power loss term during forward conduction.
Synchronous rectification circuits replace the Schottky diode with a pass transistor to perform the rectification during forward conduction as well as the current block function during reverse current conditions. Transistors employed as the rectification element exhibit much less forward current power dissipation, but measures must be taken to control the conduction state of the transistor during reverse current conditions. Allowing reverse current to flow through the synchronous rectifier from the load increases power loss dissipated by the synchronous rectifier.
Prior art synchronous rectification circuits provide synchronous control of the main and pass transistors for a majority of the current cycle, however, fail to provide adequate control during the transient states of the main and pass transistors. Prior art rectification circuits allow both the main and pass transistors to be conductive at the same time during a portion of the current cycle, allowing reverse current, or shoot-through current, to flow during the transient states. Prior art boost converters allow shoot-through current to flow from the output of the boost converter to ground. Prior art buck converters exhibit the same shoot-through current, except that the direction of the shoot-through current flows from the input to ground.
A need exists, therefore, for a synchronous rectification circuit which elimiates the shoot-through current for both boost and buck converter configurations, reducing the power loss caused by the converter which results in increased efficiency.